With continuous progress of technologies, users have an increasingly high requirement on speech quality. Expanding speech bandwidth is one of the main methods for improving speech quality. However, if information carried in the added bandwidth is coded in a conventional coding manner, coding bit rates would be greatly increased. Because of this, efficient transmission of a bitstream cannot be achieved due to a limitation of current network bandwidth. Therefore, a bandwidth extension technology is often used. The bandwidth extension technology makes use of the correlation between the low frequency band of a signal and the high frequency band of the signal in order to predict the wider band signal from extracted lower-band features.
After coding a high frequency band signal by using the bandwidth extension technology, an encoding side (which comprises an encoder) transmits the coded signal to a decoding side (which comprises a decoder). The decoding side also recovers the high frequency band signal by using the bandwidth extension technology. During signal transmission, because of network congestion, network fault or other reasons, frame loss may occur. Since packet loss rate is a key factor affecting the signal quality, in order to recuperate the lost frame as correctly as possible in case of a frame loss, a lost frame recovering technology has been proposed. In this technology, the decoding side uses a synthesized high frequency band signal of a previous frame as a synthesized high frequency band signal of the lost frame, and then adjusts the synthesized high frequency band signal by using a subframe gain and a global gain of the current lost frame, to obtain a final high frequency band signal. However, in this technology, the subframe gain of the current lost frame is a fixed value, and the global gain of the current lost frame is obtained by multiplying a global gain of the previous frame by a fixed gradient. This may cause discontinuous transitions of the re-established high frequency band signal at before and after the lost frame, and severe noises in the re-established high frequency band signal.